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Investigation of malt factors that influence beer production and quality

Van Nierop, Sandra (2005-03)

Thesis (PhD)--Stellenbosch University, 2005.

Thesis

ENGLISH ABSTRACT: A number of relevant brewing industry issues associated with malt quality were examined.
These included beer foam quality, premature flocculation of yeast during fermentation and
antimicrobial factors in malt.
The cause of poor foam at a brewery relative to other similar breweries was identified as being
related to the boiling temperature during wort preparation and the associated conformational
changes of the abundant foam protein lipid transfer protein 1 (LTPl). The temperature range
of 96 to 102°C was revealed to be critical. At the higher temperature the denaturation of LTP 1
was more extensive and its effectiveness as a foam protein was reduced. In addition, it was
shown that the prominent role of LTPI with respect to foam was as a lipid binding protein,
forming a lipid sink and protecting foam from lipid damage.
The occurrence of malt associated premature yeast flocculation (PYF) during fermentation
was induced in malt by the addition of extra-cellular fungal enzymes to the malt husk or by
micro-malting barley in the presence of fungi. In addition, treating malt husk with commercial
xylanase or adding commercial arabinoxylan to the fermentation also impacted on yeast
flocculation. It was proposed that a range of molecular weight arabinoxylans formed by the
enzymatic breakdown of the major barley husk component (arabinoxylan) resulted in PYF.
Antimicrobial activity against brewing yeast (Saccharomyces cerevisiae), other fungi and
bacteria was found in barley, malt and malt derived wort trub. Wort trub is the non-specific
precipitate of protein, polyphenols and lipids formed during wort boiling and which is, to
some extend, carried over in the wort to the fermentation. Antimicrobial activity appeared to
increase during malting. The growth of brewery collected yeast was inhibited in the presence
of brewery production wort when compared to the same wort filtered to remove the trub.
Brewery yeast was found to be more sensitive to inhibition than laboratory propagated yeast
of the same strain. Different strains of S. cerevisiae were also found to differ in their
sensitivity to inhibition. Investigation revealed that the activity originated from the inside of
the barley grain and impacted on yeast sugar uptake. However, there was no direct correlation
detected between levels of antimicrobial activity in malt and fermentation performance. At high concentrations the factors were microcidal causing cell lysis. Partial characterisation of
an antimicrobial extract from malt revealed the presence of a factor between 5 and 14 kDa,
containing a cationic peptide component. The optimum pH stability was ±5 when it was also
most cationic. The factor easily and irreversibly lost activity at extreme pH and when exposed
to certain reagents but was heat resistant in accordance with its survival in wort trub.
Preliminary results showed the presence of LTP1 associated with other peptides in the active
cationic fraction from the one malt tested.
The occurrence of malt related PYF and malt antimicrobial factors are associated with
microbial contamination of the grain. The fungi generating the PYF factors from the barley
husk while the barley's defence mechanism generates antimicrobial factors to cope with the
pathogenic effect of the fungi. In addition there is a potential link between the foam protein
LTP 1 and malt antimicrobial activity as LTP 1 or LTP 1 in association with another
component(s) is potentially antimicrobial.